In field data to correctly characterize the seismic response of buildings and bridges

Abstract

The use of in-field acquired data to characterize the seismic response of building and bridges immediately after an earthquake, is discussed in this paper; in particular, we discuss how dynamical tests could help the assessment phase and could indicate if the structure is in condition to continue its regular use. In this perspective, two different case-studies will be discussed: the first is concerned with the interpretation of a particular behavior exhibited by a group of four buildings characterized, despite of the similarities among them, by strongly different seismic response (two of them collapsed and the other two survived with a medium damage level) and the second one is concerned with a campaign of dynamical tests finalized to a possible damage detection on a r.c. bridge. Concerning the first case study it is worth noticing how, during the 6 April earthquake in L’Aquila, two couples of similar (nearly twin) buildings belonging to the same urban intervention, exhibited a dramatically different response. In the two couples, the first couple constituted by two four-story buildings and the second couple by two three-stories buildings, one element collapsed suffering the well-known soft story mechanism, while the other one survived with a damage pattern not so strong, as expected considering the similarity with the collapsed one. The in situ measurements conducted in the framework of microzoning activities provided the opportunity of gathering useful information about the dynamical properties of the soil, thus evaluating possible site amplification effects. A key role was played in this case by a local amplification of the seismic intensity, due to the geology of the site. On the same time, dynamical tests on the survived buildings enlightened their dynamic characteristics and permitted the updating of the FE models used in the structural analysis. Linear and nonlinear models of the buildings have been developed, aiming to explain the different behavior. Even the use of simple linear models taking into account the contribution in stiffness of the infilled masonry walls permitted a coherent interpretation of observed phenomena. By using more accurate nonlinear finite elements models, it was possible to reconstruct the evolution of the seismic response and to obtain a more clear unfolding of the collapse mechanism. The key role of the infilled walls has been enlightened putting into evidence how a good construction of such non-structural elements could dramatically influence the global structural behaviour. Some laboratory tests on specimens of infill walls, extracted from the collapsed building, guided the selection of the mechanical parameters of the equivalent truss elements used in the models. With the second case-study we discuss a different, interesting and effective use of dynamical measures on damaged structures, when the same measures have been conducted in a not-damaged (or reference) condition. In this case, the dynamical tests could enlighten the stiffness reduction and can help to localize the occurred damage. With this purpose some test conducted in L’Aquila (Italy) after the 6 April 2009 earthquake on the r.c. Belvedere Bridge, will be discussed.